B.de Leeuw*, M.Balemans, D.OIde Weghuis and A.Geurts van Kessel

Synovial sarcomas are soft tissue tumors that occur mainly in adolescents and young adults. The chromosomal translocation (X; 18)(pl 1.2;ql 1.2) is found in the majority of these sarcomas (1) and, as such, is thought to play a causative role in tumor formation. The occurrence of two related but distinct breakpoints in Xp 11.2 has been reported by us and others using tumor-derived somatic cell hybrids and metaphase and interphase fluorescent in situ hybridization (FISH) on primary tumor samples in conjunction with breakpoint-spanning YACs (2-5). These YACs contain several X chromosome-specific low copy repeat sequences, among which two ornithine amino-transferase-like pseudogene clusters, OATL1 and OATL2. Interestingly, we found that the occurrence of the two breakpoints correlates with the histologic phenotypes of the tumors, i.e., those with a breakpoint near the OATL1 regio